Antinuclear antibody detection is used in important tests for the diagnosis, treatment policy decision, prognosis estimation, etc. of connective tissue disease which is an autoimmune disease. The most useful method for autoantibody screening is the immunofluorescence. In this method, a patient's serum diluted to a predetermined concentration is added to human laryngeal cancer-derived epithelial cells called HEp-2 cells cultured on glass slides to cause a reaction, and then whether or not an antibody reacting with an autoantigen in the cells is present is detected using a fluorochrome-labeled secondary antibody.
A fluorescence pattern and an antibody titer (titer) are obtained as antinuclear antibody test results by the immunofluorescence. There are six main fluorescence patterns, namely, speckled pattern, homogeneous pattern, peripheral pattern, discrete speckled pattern, nucleolar pattern, and cytoplasmic pattern. The determined titer is expressed as 40-fold positive, 80-fold positive, 160-fold positive, or the like. 40-fold positive means that the fluorescence pattern is recognizable in the case of diluting the patient serum 40-fold but is not recognizable in the case of diluting the patient serum at a higher ratio (e.g. 80-fold).
The determination of an antinuclear antibody by the immunofluorescence is performed in the following two steps. In the first step, samples with a given dilution ratio (e.g. 40-fold) are subjected to a qualitative test. The qualitative test typically involves positive/negative determination visually made through microscopic observation. Samples determined as negative by microscopic examination under the fluorescence microscope are regarded as antinuclear antibody negative.
In the second step which follows, samples determined as positive in the qualitative test are further processed. Once determined as antinuclear antibody positive in the qualitative test, glass slides with the serum being doubling-diluted are prepared and a quantitative test for the antibody titer based on the dilution ratio is conducted. Here, the maximum dilution ratio giving a positive result is set as the antibody titer. The fluorescence pattern of each sample is also determined.
Patent Literature (PTL) 1 describes a method of final antibody titer measurement in the measurement of antibodies against nuclear and cytoplasmic antigens in human serum by means of indirect immunofluorescence assay. In the measurement method described in PTL 1, the final antibody titer of the patient serum is calculated based on the initial antibody titer of the patient serum and the exposure time and maximum effective exposure time (final exposure time) of the camera.